1. Field of the Invention
This invention relates to each of two coupling arrangements and, in particular, to arrangements for coupling energy between a transmission line and a waveguide.
2. Description of Related Art
Coupling of energy between a transmission line and a waveguide is usually achieved by the use of one or more wire probes or loops inserted into the waveguide cavity through the wall of the waveguide, the probes lying transverse to its axis. In the case of a waveguide accommodating circular polarization, or, alternatively, two independent orthogonal polarisations, two such probes are required which must be mutually orthogonal within the cavity and spaced a half-wavelength apart (in the direction of the axis) if high isolation and a good return loss are to be achieved. The first probe would generally be spaced a quarter-wavelength from the short-circuit end of the waveguide. Such an arrangement has two disadvantages: firstly, the probes do not have the same frequency performance, the probe further from the short-circuit having a reduced bandwidth; and, secondly, the probes are not co-planar and hence are not suitable for direct connection to a single microstrip circuit board. Isolation between the two orthogonal polarisations is improved if the structure is deliberately detuned by moving the first probe closer to the short-circuit end of the waveguide. However, in the dual probe structure such detuning results in a seriously worsened return loss because the probes are no longer tuned to the cavity.